Operational efficiency and output of a gas turbine engine generally increases as the temperature of the hot combustion gas stream increases. High combustion gas stream temperatures, however, may produce high levels of nitrogen oxides (NOx) and other types of regulated emissions. A balancing act thus exists between operating a gas turbine engine in an efficient temperature range while also ensuring that the output of nitrogen oxides and other types of regulated emissions remain below mandated levels.
Lower emission levels of nitrogen oxides and the like may be promoted by providing for good mixing of the fuel stream and the air stream before combustion. Such premixing tends to reduce combustion temperatures and the output of nitrogen oxides. One method of providing such good mixing is through the use of micro-mixers where the fuel and the air are mixed in a number of micro-mixing tubes within a plenum. In order to promote such good mixing, the same amount of fuel should be delivered to each mixing tube. This objective, however, may be challenging because fuel density is in part a function of temperature. Given such, ensuring that the fuel delivered to each tube has a uniform heat pickup may be difficult. Moreover, a significant temperature difference may develop between the mixing tubes and the outer barrel of the plenum. This temperature differential may lead to component distortion over time as well as a reduced component life.
There is thus a desire for a combustor with an improved micro-mixer design. Such an improved micro-mixer design may promote good fuel-air mixing while providing a more uniform thermal distribution across the mixing tubes and the outer barrel.